Full blood count

A full blood count (FBC) or complete blood count (CBC) is a test
requested by a doctor or other medical professional that gives
information about the cells in a patient's blood.

Automated blood count

Blood for a FBC is usually taken into an EDTA tube to stop it from
clotting. The blood is well mixed (though not shaken) and put through
a machine. The machine, called an automated analyser, counts the
numbers and types of different cells within the blood. The machine
prints out, and/or sends to a computer, the results.

Blood counting machines work by sampling blood, and sucking a standard
amount through narrow tubing. Within this tubing, there are sensors
that count the number of cells going through it, and can identify the
type of cell.

The two main sensors used are light detectors, and electrical
impedance.

Because an automated cell counter samples and counts so many cells, it
gives a very precise estimate. However, with certain abnormal cells in
the blood, they may be identified incorrectly, and not be as accurate
as a manual count.
Automated blood counting machines include the Sysmex XE-2100 and the
Celldyne range.

Manual blood count

Counting chambers, that hold a specified volume of diluted blood (as
there are far too many cells if it is not diluted) are used to
calculate the number of red and white cells per litre of blood.
To identify the numbers of different white cells, a blood film is
made, and a large number of white cells (at least 100) are counted.
This gives the percentage of cells that are of each type. By
multiplying the percentage, with the total number of white blood
cells, an estimate of the absolute number of each type of white cell
can be obtained.

Manual counting has the advantage in that it can identify blood cells
that may be misidentified by an automated counter. It is, however,
subject to human error, and has a much smaller sample size. Additional
factors, such as the quality of the blood film, also play a greater
part.

Results from a blood count

A complete blood count will normally include:

Red cells

Total red blood cells - The number of red cells is given as an
absolute number per liter.

Hematocrit

Red cell indices

(Wintrobe Indices) - Wintrobe introduced a very useful method to show
certain characteristics of red cells.

A. Mean Corpuscular Volume (MCV)

the average volume of the red cells, measured in femtolitres. This
concept utilizes the effect that the average sized RBC has on the
hematocrit. If the average RBC size is increased, the same number of
RBCs will have slightly larger cell mass and thus a slightly increased
hematocrit reading. The opposite happens if the average RBC size is
smaller than normal. The MCV is, therefore, calculated from the
hematocrit and RBC count as follows:

HCT (in %) x 10

= MCV [in cubic micrometers]

RBC Count (in millions/ L)

Reference values are 80-100 m3

If <80, cells are microcytic; if >100, cells are macrocytic.

Anemia is classified as microcytic or macrocytic based
on whether this value is above or below the expected normal range.
Other conditions that can affect MCV include thalassemia and
reticulocytosis.

B. Mean Corpuscular Hemoglobin (MCH)

The average amount of hemoglobin per red blood cell, in picograms.
This concept is an estimate of the amount of hemoglobin in the
average red cell. This is done by comparing the blood hemoglobin
level to the RBC count as follows:

HGB (in g/dL) x 10

= MCH [in picograms]

RBC Count (in millions/ L)

Reference values are 27-31 pg

C. Mean Corpuscular Hemoglobin Concentration (MCHC)

Mean cell hemoglobin concentration (MCHC) - the average concentration
of hemoglobin in the cells.
This concept estimates the concentration of hemoglobin in the
average RBC. It is different from MCH in that the average RBC
concentration of hemoglobin depends on the RBC size and the actual
amount of hemoglobin contained in the RBC. The MCHC is calculated as
follows:

HGB x 100

= MCHC [in g/dL]

HCT (in %)

Reference values are 32-36 g/dL

Examination of a well-made peripheral blood smear will give most of
the same information as will the indices. The indices are not a
substitute for examination of the peripheral blood smear, but they may
be helpful in confirming impressions. The indices are only as accurate
as the various counts and procedures that went into their calculation.

Red cell distribution width (RDW)

A measure of the variation of the RBC population

White cells

Total white blood cells - All the white cell types are given as a
percentage and as an absolute number per litre.

Platelets

Examination of Wright's Stained Peripheral Blood Smear

This procedure allows visualization of alterations in size,
shape, and structure of individual red cells and white cells, which
may have diagnostic significance in certain diseases. Examination of
the peripheral smear is essential in assessing neoplastic
hematologic disorders (e.g. leukemias), and in those disorders which
mimic neoplastic disorders. There are obviously many limitations.
For example, a peripheral blood smear cannot prove presence of
anemia per se, which must be detected by means of the hemoglobin,
hematocrit, or RBC count.

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